I'm guessing you mean the MK1 facelift headlights or the ST-170 headlights?
The two places I know of are:
www.european-parts.net
www.batinc.net
I'm not sure if BAT still carries them or not, but you could call and find out. If you order from Sascha (european-parts) be aware it will take a little while to get to you because they'll be coming from Germany.
Oh, and I'm not sure if you're aware of it or not, but the MK1 Focus RS headlights do not have the turn signals in them like the MK1 facelift headlights. They have a blanking plug in them.